1. Field of Invention
The invention relates to a light tunnel and, in particular, to a light tunnel with a gradient filter layer.
2. Related Art
Image projection system is one of the most popular products in the optoelectronic industry. The most known technologies include the cathode ray tube (CRT) technology, the digital light processing (DLP) technology, the reflective liquid crystal technology, the transmissive liquid crystal technology, and the likes. Herein, the DLP and reflective liquid crystal technologies both utilize a light tunnel for improving the luminance uniformity.
Hereinafter, a DLP is taken as an example for illustrating the function of the light tunnel in the DLP.
The DLP is digitally controlled and is operated with utilizing the light-reflecting principle. The light beam is firstly generated from a light source and then focused with a lens. After that, the light beam passes through a color filter such as including a red filter, a green filter and a blue filter. Next, the light beam achieves a digital micro-mirror device (DMD). The DMD includes a plurality of movable micro-mirrors and driving electrodes for controlling the tilt angle and rotating time of the micro-mirrors. After being reflected with the DMD, the light beam can be projected on a screen through a projection lens for generating an image.
With reference to FIG. 1, in a DLP 10, a light source 11 emits a light beam, which then passes through an UV/IR cut filter 12 and color wheel 13. After that, the light beam passes through a light tunnel 14 and a lens 15, and then reaches the reflective mirror 16. The reflective mirror 16 reflects the light beam to the DMD 17 obliquely. After processed with the DMD 17, the light beam goes to a projection lens 18 so as to be projected on a screen 19 for generating the image.
The light tunnel 14 is used to guide the light beam (e.g. to change the progressing direction of the light beam) and to collimate the light beam. In addition, the light tunnel 14 can improve the luminance uniformity and control the aspect ratio of the projected light. The light tunnel 14 is also known as a light rod, an integration rod, a light pipe or a rod lens.
Since the light beam is reflected to the DMD 17 obliquely, the paths of different parts of the reflected light beam from the reflective mirror 16 to the DMD 17 are not equivalent. Thus, the light beam may not be focused on the DMD 17, causing that the area of the light beam reaching one surface of the DMD 17 is increased. As a result, the light reaching the surface of the DMD 17 has reduced brightness and lower luminance uniformity.
In the conventional DLP 10, the DMD 17 includes a control circuit 171 for controlling the tilt angle of each DMD 17 to generate gray-scaled images according the red, blue and green gray signals of the video signals.
However, to generate image with uniform luminance, the control circuit 171 of the DMD 17 must process a pre-compensation of the light beam. This will reduce the gray level that the DMD 17 can provide, so as to affect the final image quality.
It is therefore an important subjective of the invention to provide a light tunnel with a gradient filter layer for the problem of reduced gray level caused by the pre-compensation process.